1. Field of the Invention
The invention relates to an evaporation system comprising an evaporator heated by process waste steam and a process stage heated by product vapors of the evaporator, in particular at least one further evaporator.
2. Description of Related Art
In order to operate evaporation systems as economically as possible, process heat obtained otherwise, for example in the form of waste steam obtained in the process, is used for heating the evaporator or the evaporators. Such process waste steams as obtained, for example, as waste steam of drier stages can, however, be used only to a limited extent since they are obtained at relatively low pressure or are mixed with air or inert gases. The condensation capacity of the evaporators heated with such process waste steams is as a rule reduced and the number of evaporator stages heated with such process waste steams and hence the efficiency of the evaporation system are greatly limited.
In the case of multistage evaporation systems, it is furthermore known that the product vapors of an evaporator stage of the evaporation system can be compressed by a compressor to a higher pressure in order in this way to heat a further evaporator stage with the compressed product vapors. Usually, jet compressors operated with fresh steam or mechanical compressors are used for this purpose. If mechanical compressors are used, the customary pressure increases (ratio of final pressure to intake pressure) for one-stage radial fans used in practice are 1.3 and for one-stage radial compressors are 2.5. Since the increases in vapor temperature are comparatively small, the number of evaporator stages connected in series in the case of conventional, multistage evaporation systems with vapor compression is greatly limited. Products having a high boiling point shift cannot be evaporated using this process or can be evaporated only to an insufficient extent, i.e. in the low concentration range.